The present invention relates generally to digital audio broadcasting (DAB) and other techniques for transmitting and receiving information in a communication system.
Proposed systems for providing digital audio broadcasting (DAB) in the FM radio band are expected to provide near CD-quality audio, data services, and more robust coverage than existing analog FM transmissions. However, until such time as a transition to all-digital DAB can be achieved, broadcasters require an intermediate solution in which the analog and digital signals can be transmitted simultaneously within the same licensed band. Such systems are typically referred to as hybrid, in-band on-channel (HIBOC) DAB systems, and are being developed for both the FM and AM radio bands.
In order to prevent significant distortion in conventional analog FM receivers, the digital signal in a typical FM HIBOC DAB system is, e.g., transmitted in two sidebands, one on either side of the analog FM host signal. Current FCC regulations require that the digital signal be transmitted. at a power level 25 dB below the host signal. A particularly significant source of interference in such a system is known as first adjacent analog FM interference. This interference results when a portion of an adjacent FM host carrier overlaps in frequency with a portion of a digital signal sideband. Current FCC regulations permit this type of interference to be up to 19 dB above the digital signal.
Although first adjacent analog FM interference, when present, typically affects only one of the two digital sidebands, it nonetheless represents a limiting factor on the performance of DAB systems. If conventional equal gain combining is used to process the digital signals of the two sidebands, the presence of a strong first adjacent interference signal will significantly degrade the performance of the digital signal transmission, even when one of the two sidebands is free from interference. A combined channel code in this case is overwhelmed by the high interference level in one sideband. A known approach which may be viewed as xe2x80x9chard selection combiningxe2x80x9d simply erases or discards the entire interfered-with sideband in the presence of a strong first adjacent interference signal, and uses only the other sideband. However, such a solution is less than optimal in that it discards an entire sideband, even though only a relatively small portion of that sideband may be affected by the interference. Additional techniques are therefore needed for providing improved DAB system performance in the presence of first adjacent interference and other similar types of interfering signals.
The present invention provides methods and apparatus for processing received information in digital audio broadcasting (DAB) and other communication system applications; in the presence of non-uniform interference. The received information may be, e.g., digitized audio, video or image information, data, or combinations of these and other types of information. In an illustrative embodiment, first and second digital sidebands are transmitted on either side of an FM or AM host carrier signal in a hybrid in-band on-channel (HIBOC) DAB system. The compressed digital audio information in the sidebands is encoded using an outer code, e.g., a cyclic redundancy code (CRC), and an inner code, e.g., a complementary punctured pair convolutional (CPPC) code. A receiver generates an error indicator based at least in part on a first decoding of the received information. The error indicator characterizes interference associated with at least one of the digital sidebands. The receiver then generates at least one alternative decoding of the received information if the error indicator has a designated characteristic. For example, the receiver may generate such an alternative decoding by eliminating from consideration in the receiver decoding process a designated portion of at least one of the digital sidebands.
The error indicator in the illustrative embodiment may be indicative, of the presence or absence of a first adjacent interference signal of a particular level within a frequency band associated with one of the sidebands, and may be based on, e.g., a signal-to-noise ratio measurement generated from a pilot tone associated with a given sideband, an error flag generated using the CRC outer code, statistics relating to the generation of CRC error flags in a particular part of the sideband over time, or a combination of these and other indicators. Portions of the given sideband may be eliminated from consideration via successive erasures of sideband components, i.e., subbands, starting from a component nearest the interference, thereby resulting in an increase in the effective code rate of the inner code. If none of the successive erasures satisfy the CRC, the error flag may be used to trigger an error mitigation algorithm in a corresponding audio decoder.
The invention can be applied to other types of digital information, including, for example, data, video and image information. In addition, the invention may be implemented in numerous applications other than FM and AM HIBOC DAB systems, such as Internet and satellite broadcasting systems, systems for simultaneous delivery of audio and data, etc. Alternative embodiments of the invention can utilize other types of outer codes, other types of inner codes, various types of interleaving, e.g., block interleaving, convolutional interleaving or random interleaving, and a wide variety of different frame formats, including TDM, FDM or CDM frame formats. Moreover, the invention is applicable not only to perceptual coders but also to other types of source encoders using other compression techniques over a wide range of bit rates.